The present invention generally relates to a probe apparatus for electronically sensing a parameter of a fluid. In a specific embodiment, the present invention relates to a probe which can sense temperature with a fast response time, while having a construction which is simple and yet robust. Such temperature probes have many uses, such as for sensing air temperature in a controlled heating, ventilation and air conditioning (“HVAC”) system.
Electronically based temperature sensors are well known in the art. In a typical construction, a temperature sensor includes a sensing element wired into an electrical circuit. The sensing element may be a thin strand of metal (such as a platinum resistive temperature detector or “RTD”), a thermocouple, or, commonly, a thermistor which changes its electrical resistance based on its temperature. In any event, the electrical response of the sensing element changes as a function of its temperature, such that the electrical circuit may be monitored to determine the temperature of the sensing element.
Because the sensing elements are typically somewhat fragile and delicate, the sensing elements are commonly housed in a generally rigid probe housing. The probe housing also serves to properly position the sensing element relative to the fluid flow. The rigid probe housing may be, for instance, a metallic tube or sheath.
In manufacturing assembly of the probe, the probe housing is closed at its distal end, and the sensing element and its electrical circuit is threaded into the open proximal end of the probe housing and through its length. The sensing element is thus positioned within the probe housing near the closed distal end, with wires (i.e., metal conductors within dielectric sheaths) extending the length of the probe and out of the open proximal end. The electrical resistance between the wires is indicative of sensed temperature. Once the probe is installed in the field, the wires then provide leads for the temperature probe to be electrically connected into a circuit such as a control circuit. During and after installation, the probe housing protects the sensing element from damaging contact.
After properly positioning the sensing element and wires within the probe housing during manufacturing assembly, the sensing element and wires are secured at their desired position. A common method of securing the wires/sensing element within the probe is through a curing epoxy. The thermistor may be encapsulated such as in epoxy within the sheath. The epoxy encapsulation ensures a good thermal conductivity connection between the sheath and the thermistor. The epoxy encapsulation also helps prevent damage to the thermistor due to handling of the probe. For instance, the epoxy encapsulation may extend over the final two inches or so on the distal end of the temperature probe.
The proximal end of the sheath may be also sealed such with an ultraviolet cured epoxy seal. For instance, epoxy may be flooded into the proximal end of the probe so the epoxy fills the gap between the wires and the inside diameter of the sheath along at least some length of the probe. The epoxy can then be cured (such as with exposure to UV radiation), thereby sealing the wires in place within the probe. The epoxy thus prevents the wires from rattling around within the probe during installation and use of the probe. With a good epoxy seal, the epoxy will also provide strain relief so pulling on the exposed ends of the wires will not remove the wires from the probe or otherwise damage the connections between the wires and the sensing element.
In some sensing systems, fluid flow lengthwise within the probe housing may not be problematic. In many sensing systems, however, fluid flow lengthwise within the probe is very undesirable. If the fluid pressure being measured is higher or lower than ambient, fluid flow lengthwise within the probe could represent a leak in the system. Closing the distal end of the probe housing provides a significant barrier to prevent fluid flow within the housing along its length. The cured epoxy commonly provides another level of protection to minimize or prevent fluid flow within the housing.
Because they are relatively robust and perform satisfactorily for many applications, the closed end metal housing/epoxy secured types of probes have gained widespread acceptance. However, further improvements can be made in constructing probes which make the probes perform better, such as having a faster response time. Savings can be made to reduce the cost of materials and manufacturing costs of the probes, making the probes less expensive. Improvements can be made for lower failure rates, and to make the probes less likely to be damaged in the field.